Axis labels with geom_sf & ggmap

Question

I'm trying to create a map using a basemap pulled in using ggmap (get_stamenmap) with some shapefiles plopped on top (geom_sf). I want to change axis labels.

1. When I only plot my basemap, I get axes labeled with lat/lon and NO characters - i.e. -19

2. When I add in the shapefiles using geom_sf I get axes labeled with lat/lon AND characters - i.e. 19°S

3. When I try change axis labels with scale_x_discrete along with sf_coord(expand = F) I can get what I want

4. When I add scale_y_discrete with label text as I want them I keep getting this error:

Error: Breaks and labels along y direction are different lengths

Are there invisible breaks on the y axis that I'm missing?

This works:

ggmap(SA) +
  geom_sf(data = traj_outSF, alpha = 0.4, inherit.aes = F) +
  coord_sf(expand = FALSE) +
  xlab(expression(paste("Longitude (", degree,"E)"))) +
  ylab(expression(paste("Latitude (", degree,"S)"))) +
  scale_x_discrete(breaks = c(33.5, 34, 34.5, 35, 35.5), labels = c("33.5", "34", "34.5", "35", "35.5"))

and gets me this: correct x axis

As soon as I add in scale_y_discrete I get the error message

ggmap(SA) +
  geom_sf(data = traj_outSF, alpha = 0.4, inherit.aes = F) +
  coord_sf(expand = FALSE) +
  xlab(expression(paste("Longitude (", degree,"E)"))) +
  ylab(expression(paste("Latitude (", degree,"S)"))) +
  scale_x_discrete(breaks = c(33.5, 34, 34.5, 35, 35.5),
                   labels = c("33.5", "34", "34.5", "35", "35.5"))
  scale_y_discrete(breaks = c(20, 19.5, 19, 18.5, 18,17.5),
                   labels = c("20","19.5","19","18.5","18","17.5"))

Heres a reprex:

g = st_sfc(st_point(c(34,-19)))
st_crs(g) <- 4326

SA <- get_stamenmap(bbox = c(33.18, -20.3, 35.8, -17.3), maptype = "toner-lite", zoom = 11)

ggmap(SA) +
  geom_sf(data = g, inherit.aes = F) +
  coord_sf(expand = F) +
  scale_x_discrete(breaks = c(33.5, 34, 34.5, 35, 35.5),
                   labels = c("33.5", "34", "34.5", "35", "35.5")) +
  scale_y_discrete(breaks = c(20, 19.5, 19, 18.5, 18,17.5),
                   labels = c("20","19.5","19","18.5","18","17.5"))

Answer 1

I've repeated your example and I wonder why do you need to add additional scale properties when everything works fine even without additional scale settings.

As far as I understand you are getting this error message because your y values are below zero (because your object is located in the Southern Hemisphere). Moreover you're trying to control a continuous scale with discrete settings (but this is not necessary). So you need to change the breaks argument in the scale_y_... to negative values.

library(sf)
library(ggmap)

# I've changed the way you creating sf object to remove a warning
g = st_sfc(st_point(c(34,-19)),
           crs = 4326)

SA <- get_stamenmap(bbox = c(33.18, -20.3, 35.8, -17.3),
                    maptype = "toner-lite",
                    zoom = 7) # zoom 7 to download less tiles

# The map itself
ggmap(SA) +
  geom_sf(data = g, inherit.aes = F) +
  coord_sf(expand = F) +
  scale_x_continuous(breaks = c(33.5, 34, 34.5, 35, 35.5)) +
  scale_y_continuous(breaks = seq(-17.3, -20.3, by = -.25))

So the result is below. I've removed an additional labels argument from scales to keep the coordinates format.